1. Field of the Invention
The present invention relates to a method and an apparatus for producing tofu automatically.
2. Description of the Related Art
In Japan, tofu (soybean curd) has been manufactured from ancient times. In a traditional method of tofu production, soaked soybeans are first added with water and ground to provide a soybean slurry. The resulted soybean slurry is then heated and squeezed out from a cloth filter to provide hot soybean milk. Thereafter, the hot soybean milk is added with a coagulant and stirred to initiate the solidification of the milk. The good and bad of the solidification technique produce a large difference in quality of tofu. The solidification process is thus quite important for the production of tofu.
Recently, retort packed tofu, developed for mass production of tofu, has taken the place of the traditional tofu production method which requires an artisan's skill. Retort packed tofu is of the type called Kinu-tofu, because it has a smooth taste like silk as represented by the Japanese term Kinu. For producing Kinu-tofu, cold soybean milk, mixed uniformly with a coagulant, is filled in a container and sealed. Thereafter it is heated to a high temperature for solidification.
When coagulants such as a natural bittern (main components being magnesium chloride and calcium chloride) and quick-acting, formulated magnesium chloride and calcium chloride are used, it is difficult to restrain a solidifying reaction for a long time, even though the soybean milk is kept cold. It may often happen that the solidifying reaction progresses before the soybean milk is filled into a container or a shaping box, in which instance the end product tofu, though looks solidified nicely, becomes collapsible and presents a rough taste instead of a desired bouncy and smooth one.
In recent years, coagulant adding methods called in-line mixing methods have been proposed in which a coagulant solution is poured directly into a line and mixed with cold soybean milk flowing through the line. These methods are advantageous in that they significantly shorten the time from the addition of the coagulant solution to the filling of the soybean milk into a container. However, the amount of coagulant added during the shortened time is as little as 0.1 to 2.0 wt. % per 100 g of soybean milk. If the coagulant solution added to the soybean milk is not in a high concentration of 30% or more, protein in the milk begins solidifying, thereby causing the resultant tofu to have a rough taste. If the coagulant is added in excess, the resulting tofu will have astringency and/or bitterness which adversely affect a targeted flavor thereof. Thus, the amount of the added coagulant must be controlled precisely but this is difficult to achieve with the proposed in-line mixing methods.
In addition, unless the coagulant added to the soybean milk is uniformly dispersed in a short period of time, there arise variations in the concentration of the coagulant, which act as a bar to the manufacture of homogenized tofu products. The concept of continuously adding a coagulant into a line of tofu production and mixing them therein has not been so far applied to a line of mass production of tofu in an effective way. Failure to realize such an application is due to the unreliability of a system for adding a coagulant to soybean milk and stirring them.